Well survey method and apparatus



Dec. 4, 1945. R. E. FEARON WELL SURVEY METHOD AND APPARATUS Original Filed March 25, 1940 3mm Jiafiefll? wrap Patented Dec.4,1945 1 2,390,433

UNITED STATES PATENT OFFICE 2 2,390,433 I WELL SURVEY METHOD AND APPARATUS Continuation of application Serial No. 325,880,

March 25, 1940. Serial No. 547,153

This application July 29, 1944,

17 Claims. (Cl. 250-83.!

This invention relates to geophysical prospecting and more particularly to a method and apparatus for determining the nature of formations lying adjacent to openings extending downwardly into the earth such for example as the drill holes made to obtain oil.

This application is a continuation of applicants pending application, Serial Number 325,- 880, filed March 25, 1940, for Well survey method and apparatus.

Surface prospecting, that is the determination of the nature of geological formations lying on or close to the surface of the earth does not present the same problems as are presented by attempts to determine the nature of subsurface strata, for the surface strata are relatively available and can be sampled and observed without great difllculty. Therefore, although the principles of the present invention may be applied to surface prospecting or applied to the examination of samples either obtained on the surface or by core sampling during the drilling process, the present invention is more particularly adapted to the geophysical exploration of subsurface strata in situ, that is by the examination of the strata without moving it from its original, relatively inaccessible location far below the surface of the earth.

Numerous methods and devices have been proposed for determining the nature of subsurface strata without actually sampling it and many of the proposed devices have been adapted to be lowered into drill holes to measure characteristics of the strata from positions adjacent thereto in the bore holes. Among the methods and devices that operate in this manner have been devices which measure the natural radioactivity of the strata adjacent the drill hole at various depths.

- Devices have also been proposed for exposing the surrounding trata to primary radiations which in turn induce secondary radiations in the surrounding strata or are scattered and returned to the drill hole so that measurements can be made of the secondary or scattered radiations.

Up to the present time the nature of the scattered or induced radiations has apparently not been considered in any great detail and no attempt has been made to do more than suggest the measurement of their intensities. It has now been discovered that the measurement of the quality or kind of radiations that return to the drill hole under certain stimuli or from natural causes, can be made and that these measurements will yield information not heretofore readily obtainable about the surrounding strata.

Assuming for the moment that the necessary measurements can be made, let us assume that a formation is bombarded by neutrons. In such a case, aside from the natural radiations emanating from the formation, there will be a certain intensity of neutrons that are scattered by the formation and reach whatever measuring instrument is used. This scattering of neutrons is a ballistic process in which the neutrons, which are actual particles moving in relatively high speeds, simply bounce off of the atoms of the formation.

In addition to the scattering of the neutrons, however, there is a second phenomenon that occurs, namely the absorption of a certain proportionof the neutrons by the molecules of the formation. When neutrons are absorbed a certain amount of energy is given of! in the form of gamma radiations and a portion of these gamma radiations, as well as a portion of the scattered neutrons, will reach the detecting instrument.

In the case of the scattering of the neutrons, the phenomenon being caused largely by the collision between neutrons and atoms, the scattering will be more closely related to a gross physical characteristic like density than to the more specific nuclear characteristics of the formation. In the case of the gamma ray generation, however, the absorption of the neutrons in atoms of the formation and emission of gamma rays will depend to a much larger extent upon the specific chemical nature of the atoms making up the formation and hence a measure of the gamma rays generated will be more a measure of the chemical nature of the formation.

In order to embody the above concepts in a device and method that would be usable for practical purposes in actual field surveying operations,

an apparatus has been devised which consists of a relatively large metal casing or capsule of tubular shape adapted to be lowered into a drill hole and suspended therein by means of a cable carrying the necessary electrical conductors to connect measuring instruments which are carried therein to the recording instruments which are,

maintained on the surface of the ground.

Within the capsule is positioned a source of neutrons which will radiate neutrons into the strata surrounding the drill hole so as to cause the desired emission of neutrons and gamma rays thereirom. Also within the capsule is positioned a detector of gamma rays and a detector of neutrons, both carefully shielded from the primary source of neutrons so that neutrons emitted therefrom cannot directly effect them.

In the usual embodiment the measuring instrument consists of a pair of ionization chambers differently shielded and having the same or different kinds of gas therein at the same or different pressures so that one of the ionization chambers is more sensitive to neutrons than to gamma rays and the other ionization chamber is more sensitive to gamma rays. With such instruments, even though neither will measure its own type of rays to the entire exclusion of the other, a measure of each typ of radiations can be made by properly combining the outputs of the two ionization chambers. Thus, if we make A=the measurement made by the ionization chamber more sensitive to gamma rays, H=the measurement made by theionization chamber more sensitive to neutrons, N=the measurement that the neutrons alone would produce in the chamber more sensitive to them and 1=the measurement that the gamma rays would produce in the chamber more sensitive to them, then properly amplifying and combining the measurements made by the pair of ionization chambers it has been discovered that accurate measurements of the two types of radiations can be made. A consideration of the accompanying drawing and th following detailed description of the pre-' ferred embodimentof this invention will reveal many additional details of the method and apparatus by which it may be practically applied and also serve to show additional advantages for their use. 1

The single figure of the drawing is a diagrammatic illustration of the preferred form of this invention. .As illustrated, the preferred embodiment of this invention consists ofa main container or capsule l adapted to be lowered into a drill hole at the end of a supporting cabl II which carries insulated electrical conductors of a sumcient numbet to transmit the results of the measurements produce neutrons of a slower type which is often desirable because they are more readily absorbed responsive to gamma radiations and the ionization chamber l8 being designed to be more responsive to neutrons. Toefiect this defferenc in responsiveness the ionization chamber I is preferabiy filled with argon at around 1350 pounds per square inch pressure and is practically unshielded, that is the sheath is only thick enough to prevent the entrance of the alpha and beta rays.

10 where k, K, K' and K" are constants. Thus by.

The ionization chamber It, on the other hand, preferably contains hydrogen at around 900 pounds per square inch pressure and is shielded quite heavilyfby a lead outer casing l6 4 or 5 centimeters in thickness. Each ionization chamof potential and the inner electrodes I! are connected to the positive side of the battery through separateresistors 20 and 2|, respectively; The

positive side of the battery is preferably grounded through a connection 22 to the casing l0. Thev resistors 20 and -2l will preferably have resistances of the order of 10 ohms and the and produce a larger'proporti'on of gamma rays Y from the surrounding strata. vWhen the source of faster neutrons is employed the neutrons may be slowed down by,,enclosing the neutron producing source with an hydrogenaceous material such 'as parafiin I211. The thickness of this surround ing layer can be adjusted to slow down the averag speed of the emitted neutrons without completely preventing their-emission into the surrounding strata. A lead shield l2b around said primary source of neutrons and hydrogenaceous material to prevent the emission of gamma rays is also desirable since it eliminates gamma rays that would otherwise be scattered or reflected and recorded. The paraflin and the lead shield should diations and from any neutrons that start towards the measuring instruments from the primary sources of radiation. Paraflin has been found to be satisfactory as the hydrogenaceous material for this purpose. and three or four centimeters each of paraffin and lead make a very satisfactory shield by acting to considerably suppress neutrons and any stray gamma rays. The order in which they are placed above the primary source of ra-,

diation is not important.

On the opposite side of the sheld from the primary source of radiation and within the capsule I. are two ionization chambers l5 and IS, the

' ionization chamber l5 being designedto be more respective potentials developed across them'w'ill vary indirect relationto the currents whichthe impinging radiations cause to flow' in the ionization chambers l5 and Hi. Across each of'the resistors 20 and.2l is laced an amplifier, 23'and 24 respectively, each serving to multiply the measurement indicated by the potential drop across the resistor to which it is connected bya constant factor which corresponds to K in the formulas previously given. For convenience in this in-'I stance the circuit elementsare adjusted whereby 25 and 2B'respectively. Since the original voltage develope'd across resistor 20 corresponds to A in the formulas reviously given the voltage across the resistor 25 will correspond to KA in the formulas and the voltage across resistor 26, since that across resistor 2| corresponded to H-in the formulas, will correspond to KH; A second pair of amplifiers, 21 and28, are connected across resistors 25 and 26 respectively, amplifier 21 multiplying the measurement indicated by the voltage across resistor 25 by a factor corresponding to the constant K' in the formul and amplifier 28 multiplying the measurement indicated by the voltage across resistance 26 by a factor corresponding to K" in the formula. Resistances 29 and 30, respectively, are then placed across the outputs of amplifiers 21 and 28 and the voltages developed thereacross correspond to AKK' and HKK" respectively. By means of a pair of leads 31 extending from one end of the resistor 30 which from the opposite end of the resistor 25 which develops a voltage corresponding to AK, a voltage is obtained which corresponds to the formula 'AK- HKK or to 7' which isthe gamma'ray in- .tensity. JSimilarly by use of apair of leads 32 connected to one end of: the resistor 26 which develops a voltage corresponding to HK' and to Thecurrents so obtained are carried to the sur- I face through the cable ll over a'measuring wheel 33 and on to the cable reel 3|. From the cable reel they are taken through slip rings 35 and brushes 36, amplified if necessary by further amplifiers 31 and recorded side by side on a recorder 38. This recorder is driven by suitable gearing 39 or by an electrical transmission system such as a Selsyn transmission system from the measuring wheel 33 so that the movement of the recorder tape corresponds to the movement of the measuring capsule up and down in the drill hole. Thus the record made correlates the record of the measurements made with a record of the depths at which the measurements were taken.

Obviously, a very considerable number of modiflcations in the apparatus and method described may be made within the scope of the present invention by those skilled in the art. For example, the primary source of radioactivity may be omitted and the measurements made only of natural radioactivity. Alternatively, measurements may be made both with and without the primary source of radioactivity and the measurements comparedto give a measurement of induced or scattered radioactivity apart from any measurement of natural radioactivity. Further the design or shielding of the ionization chambers may be varied and the various amplifying units may be positioned on the surface rather than in the main casing III. Still further, the measuring currents may be commutated and transmitted to the surface as alternating currents as disclosed in patent application Serial Number 279,577 filed June 16, 1939, by Serge A. Scherbatskoy, or a null system of measurement may be used such as described in patent application Serial Number 299,767 filed October 16, 1939, by Serge A. Scherbatskoy. As another modification the lag in measurements caused by the capacity of the ionization chambers may be overcome by the application of the principles of the invention disclosed in this inventors application Serial Number 311,217 filed December 27, 1939. Also, the power for operating the device and the currents indicating the results of the measurements may be transmitted from and to the surface by means of a plurality of carrier frequencies travelling over a single conductor as described in this inventors application Serial Number 311,219 filed December 27, 1939. Any or all of these modifications and numerous others thatwill readily occur to those skilled in the art are contemplated as a part of this invention.

The constants designated K, K and K" may be established and the amplifiers set to give the desired degree of amplification by laboratory operation of the device under controlled conditions of exposure to neutrons and gamma rays. In making these adjustments the resistors 20, 2|, 25, 26, 29 and 30 may be adjusted as necessary as well as the amplifiers themselves and the voltages impressed across the ionization chambers may be adjusted and ma be made different from each other as necessary. The type and pressure of the gases in the ionization chambers and their shielding is also subject to variation to fit the instrument for the particular type of measurements that it may be desired to make. All this is contemplated and believed well within the skill of those familiar with this art. Obviously, a measurement may be made of gamma rays or of neutrons alone and it is not necessary to make both measurements at the same time although this has been found highly desirable,

I claim:

1. A method of geophysical prospecting that comprises at various levels from a well bore or other opening in the earth bombarding the ad- Jacent strata with neutrons, simultaneously at substantially the same level separatel measuring the gamma rays and neutrons returning to the well bore from the strata, simultaneously determining the depths at which said measurements are made and recording said measureidnergs in correlation with said determinations of 2. A method of geophysical prospecting that comprises at various levels from a well bore or other opening in the earth bombarding the ad- Jacent strata with slow neutrons, simultaneously at substantially the same level, separately measuring the gamma rays and neutrons returning to the well bore from the strata, simultaneously determining the depths at which said measurements are made and recording said measurements in correlation with said determinations of depth.

3. A method of geophysical prospecting that comprises, from a continuously varying depth in a drill hole or other opening in the earth, bombarding the adjacent strata with neutrons while simultaneousl and at the same depth separately measuring the neutrons and gamma rays returning to said bore hole or opening in the earth, continuously determining the depth at which the measuring is being done and continuously recording the measurements in correlation with the determination of depth.

4. An apparatus for geophysical prospecting that comprises a sealed chamber adapted to be lowered into a drill hole or other opening in the earth, and, contained within said chamber, a primary source of neutrons, a detector for gamma rays, a detector for neutrons, and a shield between said primary source of neutrons and said .detecting instruments for preventing the direct emission from said primary source from reaching said detecting means.

5. An apparatus for geophysical prospecting that comprises a sealed chamber adapted to be lowered into a drill hole or other opening in the earth and, containing therein, a primary source of neutrons, a detector for gamma rays, a detector for neutrons, a shield between said primary source of neutrons and said detecting instruments for preventing the direct emission from said primary source from reaching said l.'letecting means and means for amplifying the outputs of said detecting means and transmitting said outputs to the surface for recording.

6. An apparatus for geophysical prospecting that comprises a pair of ionization chambers one of which is relatively more responsive to gamma rays and the other of which is relatively more responsive to neutrons, interconnected amplifying means for said two ionization chambers so connected as to amplify the measurements made thereby and at the same time to correct the: measurement made by the neutron sensitive chamber so as to substantially exclude any error due to the efiect of gamma rays on said chamber and to correct the measurement made by the gamma ray sensitive chamber to eliminate the eflfect of neutrons on this measurement.

7. An apparatus for geophysical prospecting that comprises a sealed capsule adapted to be lowered into an opening in the earth, a, primary source 'of neutrons in said capsule, hydrogenaceous material surrounding said primary source of neutrons to lower their speed, a shield also surrounding said primary source of neutrons to bers, two primary resistors, one connected to the other electrode of each of said ionization chambers and having its opposite end connected to the opposite terminal of said source of potential, two primary amplifiers, one having its input connected across each of said resistors, two secondary resistors, one connected across the output of each of said primar amplifiers, two secondary amplifiers, one having its input connected across each of said secondar resistors, two tertiary resistors, one connected across the 'output of each of said secondary amplifiers, the arrangement being such that the secondary resistor for one amplifier set is connected in series with the tertiary resistor for the other amplifier set so that differential voltages may be taken from the circuit representing the difference between the output of the primary amplifier for one of the primary resistors and the output of the secondar amplifier for the other primary resistor and vice versa, a cable for supporting the capsule in the well and conveying these currents to the surface, a recorder, for recording said currents and means for driving said recorder in accordance with the motions of the cable up and down in the well bore or other opening.

8. A method of geophysical exploration that comprises exposing a subterranean formation to a primary neutron source whereby a stream of neutrons bombards said formations, making a measurement that is predominantly influenced by secondary neutron emission of the formation to be studied, making a second measurement that is predominantly influenced by gamma ray emission of the formation to be studied and combining said measurements to give a measurement more accurately indicative of the neutron emission of said formation than the original measurement.

9. A method of geophysical exploration that ,comprises exposing a formation to bombardment ,by a primar stream of neutrons, making a measurement that is predominantly influenced by the secondary neutron emission of the formation to be studied, making a second measurement that is predominantly influencedby gamma ray emission of the formation to be studied and combining said measurements to give two measurements one more accurately indicative of neutron emission of said formation than the first measurement and the other more accurately indicative of the gamma ray emission of said formation than the said first measurement.

10. A method of geophysical exporation that comprises at various levels from a well bore or comprises bombarding a geological formation with neutrons and simultaneously but separately measuring secondary neutron and gamma ray Emission of the formation as an index of its naure.

12. Method of geophysical prospecting that comprises bombarding a geological formation with neutrons, simultaneously but separately measuring secondary neutron and gamma ray emission of the formation as an index of its nature and recording the measurements in correlation with indications of the place of measurement.

13. A method of geophysical prospecting that comprises at various levels from a cased well bore or other metal sheathed earth opening bombarding the adjacent strata with neutrons, simultaneously at substantially the same level separate- 1y measuring the gamma rays and neutrons returning to the well bore from the strata, simultaneously determining the depths at which said measurements are made and recording said measurements in correlation with said determinations of depth.

14. A method of geophysical prospecting that comprises at various levels from a cased well bore or other metal sheathed earth opening bombarding the adjacent strata with slow neutrons,

simultaneously at substantially the same level, separately measuring the gamma rays and neutrons returning to the well bore from the strata, simultaneously determining the depths at which said measurements are made and recording said measurements in correlation with said determinations of depth.

15. A method of geophysical prospecting that comprises, from a continuously varying depth in a cased drill hole or other sheathed opening in the earth, bombarding the adjacent strata with neutrons while simultaneously and at the same depth separately measuring the neutrons and gamma rays returning to said bore hole or opening in the earth, continuously determining the depth at which the measuring is being done and continuously recording the measurements in correlation with the determinations of depth.

16. A method of geophysical prospecting that comprises at various levels from a well bore or other opening in the earth bombarding the adjacent strata with radiation which will produce nuclear disintegration therein, simultaneously at the same level separately measuring two different f types of radiation, said radiations having different penetrating characteristics returning to the well bore from the strata and resulting from said nuclear disintegration, simultaneously determining the depths at which said measurements are made and recording said measurements in corthe well bore from the strata and resulting from said nuclear disintegration, simultaneously determining the depths at which said measurements are made and recording said measurements in correlation with said determinations of depth.

ROBERT EARL FEARON. 

